Decoration of ceramic, enamel or glass objects mostly proceeds by providing a master image, monochromic or full colour, on a substrate with releasing properties, by transferring said master image to the ceramic, enamel or glass object and by firing said objects to fix the image on said ceramic, enamel or glass object.
The master image is mostly made by screen-printing. In screen printing it is necessary to produce various screen images for the various colours to be printed, which makes the technique quite time-consuming and for small edition or personalised work totally uneconomical. By decoration is mostly to be understood the application of identification printing on ceramics or glassware, e.g., a label, a bar-code, etc. Moreover, the screen printing process uses liquids, mostly organic and volatile, and requires thus measures to avoid organic vapours to reach the environment. Further, screen printing is basically an analog process while nowadays more and more information and images to be printed become available in digital form.
The need for a dry and digital method for producing master images for decorating ceramic of glass objects is thus real.
In EP-A 097 134, it has been disclosed to decorate tiles by electrostatographic methods. Electrically charged dry colouring ceramic pigment particles are image-wise deposited onto a photoconductive drum and transferred to a tile. The ceramic material contains no binder resin, and therefore quite complex transfer and bonding aids have to be applied to the tile to assist the transfer and the bonding of the ceramic pigment to the tile in order to be able to flawlessly further process the tile. In this document it is disclosed to produce multi-colour images by using a machine printing for each colour. Apart from the need for complex transfer and bonding aids on the tile, the method according to this disclosure presents some additional problems: first, the contact between photoconductor and tile is rather unstable since an attempt is made to contact two hard surfaces, secondly the photoconductor is very likely to be damaged, since it is a fragile member whereas the tile is hard, thirdly, since the contact is unstable, difficulties arise to register the colour image. Finally, add-on transfer of different colours on already deposited colour layers will be critical in the unstable contact. Apart from cited problems, the disclosure is limited to the decoration of flat tiles and is not useful for decorating essentially non-flat objects.
In DE-A 40 41 358 it is disclosed to use master images made by electrophotography to decorate ceramic, enamel or glass objects by transferring said master images to said objects. The method includes the steps of providing an image-wise deposition of toner particles exhibiting ceramic colouring characteristics on an intermediate substrate having release properties, and transferring said master image to an object by first coating the master image with a collodion lacquer. There is, however, in this disclosure no special attention given to the electrophotographic process and the disclosure hints to the fact that any photocopying machine, whatever the imaging method, fixing method, transporting method of the master image, the method can provide good images. It is in anyway necessary to fix the toner particles to the intermediate, since a non-fixed toner image can barely be handled. It is furthermore disclosed that said process is only workable when the toner particles contain, apart from a binder resin, either a metal, a metal alloy and/or a metal-oxide. It is disclosed that only by using such toner particles and pigments a master image can be created giving, after transfer and firing to the object to be decorated, enough optical density and sharpness of the final image. By limiting the useful pigments to either a metal, a metal alloy and/or a metal oxide, the method disclosed in DE 40 41 358 has a limited practical use, as within the type of pigments enumerated only a few colours can be provided.
In EP-A 647 885 an electrophotographic method, essentially equal to the method disclosed in DE-A 40 41 358, for the production of master images for the decoration of ceramic objects is disclosed. The problem obtaining high optical density with toner particles comprising ceramic pigments (i.e. mostly metal oxides) is addressed by laying very thick toner layers (between 0.3 and 0.6 mm thick, whereas a normal full colour electrophotographic image comprises toner layers of 0.01 mm thickness). These thick toner layers can give problems during the fixing to the intermediate substrate, therefore a non-contact fixing step in an oven or by contacting the rear side (non-image side) of the intermediate substrate is used. In this disclosure it is said that optionally glazing material can be applied to the object to be decorated before the master image is transferred or after the master image is transferred. This document discloses further that it is essential that the toner particles contain a binder and a ceramic pigment, said pigment being preferably a spinel or zirconiumsilicate, and optionally a charge enhancing compound and some flow enhancing additives, in order to be able to obtain a high optical density. This indicates that the method disclosed, shows, even when depositing thick toner layers, a problem in obtaining enough optical density.
In JP-A 08/11496 it is disclosed to produce master images for decoration of ceramics by electrostatographic means and by using toner particles comprising both a colouring agent and glazing material.
In GB-A-2 151 189 a process for applying a pattern to a ceramic article disclosed, wherein a pattern is generated by a reprographic process and this pattern then printed on a temporary support with release properties. The printing of the pattern can proceed by a xerographic process.
In EP-A-280 378, and U.S. Pat. No. 4,943,506 white toner particles, comprising TiO.sub.2 as pigment have been disclosed. In U.S. Pat. No. 4,990,427 a black toner comprising a inorganic black pigment is disclosed.
Although the methods, disclosed in the references cited, do present a step forward when compared to methods using screen printing to form a master image, there is still room for improvement of the electro(stato)graphic method for producing master images.